In the books she ends up commanding DS9. Major Kira's position was originally written for her, but Michelle Forbes didn't want it. It surprises me that she never showed up on the show at least one time.

Erika Benteen from the Homefront/Paradise Lost two parter would have beena good addition during the final season. The events during that two parter would have meant that she was known to most of the main cast and not all of them would have favourable opinions of her leading to drumroll dramatic tension!

Bashir could have fallen for Benteen ratehr than Ezri making the romance less squick imho and helping Benteen to loosen up a bit.

Also Pel, the cross dressing female ferengi from Rules of Acquisition that Quark had feelings for, could have returned as a recurring romantic interest in the final season. JirinPanthosa already suggested that the female rights on fereginar could have been run through a different female than Ishka and Pel would have been perfect for that.